(1) Field of the Invention
This invention relates to the fabrication processes use to create semiconductor devices, and more specifically to methods used to create metal via structures, used to interconnect overlying and underlying metallization levels.
(2) Background of the Invention
The semiconductor industry is continually striving to increase the performance of semiconductor devices, while still maintaining, or decreasing, the manufacturing cost of these same semiconductor devices. These objectives have been successfully addressed by the ability of the semiconductor fabrication community to successfully create silicon devices with specific sub-micron features. The advent of micro-miniaturazation, or the use of sub-micron features, has largely been accomplished by advances in several semiconductor fabrication disciplines, specifically photolithograhy and anisotropic dry etching. The development of more sophisticated exposure cameras, as well as the use of more sensitive photoresist materials, have resulted in sub-micron images in photoresist layers being routinely obtained. In addition similar advances in dry etching, or reactive ion etching, (RIE), have allowed the sub-micron images in photoresist layers, to be successfully transferred to underlying materials, used for the construction of silicon devices. The use of sub-micron features results in decreases in parasitic capacitances, as well as resistance decreases, both providing performance benefits. In addition the smaller features allow a greater number of smaller silicon chips to be obtained from a specific size starting wafer, thus reducing the manufacturing cost for individual chips.
The use of semiconductor chips, fabricated with sub-micron features, does however create specific areas of concern, not encountered with semiconductor chips, fabricated with less aggressive designs. For example, metal filled via holes, used to interconnect metallization levels, are more difficult to form, and fill, when using sub-micron designs. Via holes, with sub micron diameters, are difficult to fill with conventional metallization deposition techniques, such as sputtering or evaporation. The high aspect ratio of the via hole, that is the depth of the via, divided by the diameter of the via opening, requires a low pressure chemical vapor deposition, (LPCVD) process for adequate via hole filling. Since it is difficult to deposit aluminum based metallizations, using LPCVD, the semiconductor industry has used LPCVD tungsten to fill these high aspect ratio via holes. However even with the use of LPCVD tungsten, several shortcomings still exist. For example since the LPCVD fill of a via hole proceeds by tungsten depositing on the sides of the via hole, a seam or imperfection can exist in the center of the tungsten filled via, at the point of convergence of the depositing layers. This seam, when subjected to RIE etch back processes, used to remove unwanted tungsten from areas of the silicon device, other than the via hole, can evolve into a larger seam or defect, making it difficult for subsequent overlying metallizations to successfully cover. Thus overlying metallizations, thin in areas overlying the enlarged seam, may experience higher current densities than desired, and possibly resulting in electromigration failure.
This invention will describe a process for creating metal vias, or studs, using the approach of initially forming the metal via, then performing the processes needed for dielectric passivation. Therefore the difficulties of forming a narrow diameter via in a dielectric layer, filling with a metal, and etching back, are avoided. Therefore this approach can use the higher conductivity, aluminum metallization, obtained using the now acceptable r.f sputtering procedure. This approach has been offered by Fisher, et al, in U.S. Pat. No. 4,917,759, and Allman, et al, in U.S. Pat. No. 5,312,512, in which emphasis is placed on filling narrow spaces between metal via studs, with dielectric material. However this invention will describe and feature a novel process in which both an underlying interconnect metallization structure, and a metal via stud, are formed from one composite metal layer, using two photolithographic, and two RIE processes.